This invention relates generally to devices and methods for inserting pedicle screws into the spine, and more specifically to devices and methods for accurately establishing a pedicle screw tap hole drilling trajectory.
The bones and connective tissue of an adult human spinal column consist of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine.
The spinal column of bones is highly complex in that it includes these more than 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art that achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis along which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles.
The pedicles are the strongest parts of the vertebrae and therefore provide a secure foundation for the screws to which the rods are to be attached. In order to obtain the most secure anchor for the pedicle screws, it is essential that the screws be threaded in alignment with the pedicle axis and not be allowed to deviate therefrom. Misalignment of the pedicle screws during insertion can cause the screw body or its threads to break through the vertebral cortex and be in danger of striking surrounding nerve roots. A variety of undesirable symptoms can easily arise when the screws make contact with nerves after breaking outside the pedicle cortex, including dropped foot, neurological lesions, sensory deficits, or pain.
Known surgical procedures to avoid misalignment of the pedicle screws involve recognizing landmarks along the spinal column for purposes of locating optimal tap hole entry points, approximating tap hole trajectories, and estimating proper tap hole depth. Some surgeons use a Kocher clamp applied to the vertebral bone for a reference mark and/or view radiographs or other medical images to better understand relative positions of the patient""s anatomy. X-ray exposures and/or fluoroscopy can sometimes be used to monitor the advancement of a pedicle screws through the vertebra. Unfortunately, these procedures are subject to surgeon visual approximation errors, and anatomical landmarks are different for each patient. Further, prolonged radiation exposure to a patient is undesirable. U.S. Pat. No. 4,907,577 (Mar. 13, 1990) discloses a jig that is described therein as providing a safe route for drilling pedicle screw tap holes, by identifying a precise location for drilling to prevent deviation from the drilling direction so as to prevent injury during surgery to the nerve root or spinal cord. However, the jig has a variety of moving parts that must be adjusted and monitored simultaneously during the adjustments, making operation of the jig difficult and time consuming. Further, operation of the jig must occur during surgery, as it must be held adjacent the vertebral body to determine the proper adjustment settings. Finally, adjustment of the jig to the proper settings requires precise visual approximation by the surgeon, an activity that should be minimized to ensure that a misaligned trajectory is not established in place of a safe one.
More technologically advanced systems such as the StealthStation(trademark) Treatment Guidance System, the FluoroNav(trademark) Virtual Fluoroscopy System (both available from Medtronic Sofamor Danek), and related systems, seek to overcome the need for surgeons to approximate landmarks, angles, and trajectories, by assisting the surgeons in determining proper tap hole starting points, trajectories, and depths. However, these systems are extremely expensive, require significant training, are cumbersome in operation, are difficult to maintain, and are not cost effective for many hospitals.
U.S. Pat. No. 5,474,558 (Dec. 12, 1995) and 5,196,015 (Mar. 23, 1993) propose a procedure in which a screw opening is started in part of a skeletal region, e.g., a pedicle of a lumbar vertebra, and an electric potential of a certain magnitude is applied to the inner surface of the opening while the patient is observed for nervous reactions such as leg twitching. The opening continues to be formed while the electric potential is applied until a desired hole depth is obtained in the absence of nervous reaction to the potential. The direction in which the screw opening is being formed is changed to a direction other than the last direction, after observing patient reactions to the electric potential when the screw opening was being formed in the last direction. Unfortunately, this procedure is inherently reactive rather than proactive, in that the surgeon becomes aware of the misalignment after the patient exhibits a nervous reaction, and by that time the misaligned hole has been drilled.
Therefore, there is a need for a simple device that eases the difficulties associated with safely placing pedicle screws. Specifically, there is a need for such a device that assists a surgeon in making more accurate the surgeon""s assessment of the proper insertion trajectory of the pedicle screw. Further, there is a need for such a device that does not require the surgeon to rely on visual approximations. In addition, there is a need for such a device that proactively determines the desirable drilling trajectory rather than reactively informing the surgeon when an improper trajectory has been used.
The needs identified above and other needs in the art are achieved by the present invention that provides a gravity dependent pedicle screw tap hole guide and methods of use thereof.
One embodiment of a gravity dependent pedicle screw tap hole guide of the present invention has a shaft with a proximal end, a distal end, a longitudinal axis, and a fluid chamber attached to the shaft. A bubble in the fluid chamber indicates whether or not the chamber is level and/or to what degree it is not level. The translucent wall of the chamber has a reference mark positioned so that that when the bubble is centered under the reference mark, the longitudinal axis of the shaft is parallel to the acting direction of gravity. The wall also has a grid that, when the bubble is not centered under the reference mark, indicates an angular difference (preferably in two perpendicular planes) between the longitudinal axis of the shaft and the acting direction of gravity. Preferably, the longitudinal axis of the shaft extends perpendicular to a plane in which a platform holding the chamber extends. The chamber is preferably a hemispherical enclosure with a central axis that is parallel to the longitudinal axis of the shaft.
In operation of this embodiment, the surgeon first exposes a vertebral bone and applies a Kocher clamp to the spinous process in a vertical position (where the longitudinal axis of the clamp is parallel to the acting direction of gravity) to his best visual approximation. Preferably, the guide of this embodiment is used here to make the vertical placement more accurate, by holding the shaft parallel to the longitudinal axis of the Kocher clamp while manipulating the shaft with the Kocher clamp so that when the bubble is centered under the reference mark, the surgeon knows that the Kocher clamp is in a vertical position.
Next, a lateral radiograph is taken and used to approximate the cephalad-caudad declination of the pedicle of interest and the medial angulation of the pedicle is determined from preoperative transaxial MRI and/or CAT scan images. The surgeon then positions the distal end of the shaft against the exposed vertebral bone in the vicinity of the base of the superior articular process and the base and middle of the transverse process (referred to herein as the xe2x80x9cpreferred tap hole entry pointxe2x80x9d), and angulates the shaft until the angular difference between the longitudinal axis of the shaft and the acting direction of gravity matches the determined cephalad-caudad declination (in the cephalad-caudad plane), and the medial angulation (in the medial plane). During this angulation, the surgeon can view the bubble""s position relative to the grid lines, to know when and in what direction additional angulational adjustment of the shaft is necessary to bring the shaft to the desired position.
Once the shaft has been placed in the desired position, the surgeon can be confident that drilling into the vertebral bone along the trajectory established by the longitudinal axis of the shaft in the desired position will result in a pedicle screw tap hole that is formed to maximize the stability of a pedicle screw subsequently screwed thereinto. The shaft can be hollow to accommodate a drill bit for this purpose, or, if the shaft is not hollow, the distal end of the drill bit can be placed against the preferred tap hole entry point of the exposed vertebral bone, and the shaft can be held parallel to the longitudinal axis of the drill bit so that the shaft and the drill bit can be angulated in parallel together until the guide indicates the drill bit is at the desired angle.
Another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention also has a shaft with an attached fluid chamber housing a level-indicating bubble that rests under a reference mark when the chamber is level. The chamber is movably attached to the shaft and thereby positionable relative to the shaft. Specifically, the degree of perpendicularity of the longitudinal axis of the shaft relative to a plane defined by the chamber can be varied in at least two planes. In this regard, the movable attachment of the chamber to the shaft is achieved by two rotatable mountings, the first being between the shaft and the second rotatable mounting, and the second being between the first rotatable mounting and the chamber. The first rotatable mounting rotates about an axis extending perpendicular to the longitudinal axis of the shaft, and the second rotatable mounting rotates about an axis extending perpendicular to the plane defined by the chamber. Each of the rotatable mountings can be secured at any position to which it can be rotated. When each rotatable mounting is in its zero position, the plane of the chamber is perpendicular to the longitudinal axis of the shaft and, accordingly, when the enclosure is oriented such that the bubble is under the reference mark, the longitudinal axis of the shaft is parallel to the acting direction of gravity. Marks on the mountings preferably indicate the relative angle of rotation of the rotatable mounting with respect to the zero position, such that if either or both of the rotatable mountings are placed in a rotated position, the user can read the marks to determine the angular difference between the longitudinal axis of the shaft and the plane defined by the chamber when the chamber is oriented so that the bubble is under the circle.
In operation of this embodiment, the surgeon proceeds as indicated above with regard to the first embodiment, but use of this embodiment to make the Kocher clamp vertical placement more accurate is as follows: The rotatable mountings are placed in their respective zero positions, and the shaft is held parallel to the longitudinal axis of the Kocher clamp while being manipulated with the Kocher clamp until the bubble is centered under the reference mark, at which time the surgeon knows that the Kocher clamp is in a vertical position.
After determining the cephalad-caudad declination and medial angulation of the pedicle of interest, the surgeon places the first rotatable mounting into a rotated position at an angular offset matching the cephalad-caudad declination, and places the second rotatable mounting into a rotated position at an angular offset matching the medial angulation. During these rotations, the surgeon can view the marks to ensure that the mountings are rotated to the desired angles. Then, the surgeon positions the distal end of the shaft against the preferred tap hole entry point, and angulates the shaft until the bubble is under the reference mark. The surgeon can then safely drill the tap hole as desired along the trajectory established by the longitudinal axis of the shaft. Again, the shaft can be hollow and/or held in parallel to the drill bit as the drill bit is angulated against the preferred tap hole entry point.
Yet another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is similar to the first embodiment discussed above, but uses an accelerometer instead of a fluid chamber housing a level-indicating bubble. An accelerometer is known in the art as an electronic device that can determine its angular orientation relative to the acting direction of gravity, and therefore can be used to determine, for any device in fixed relation to the accelerometer, the angular orientation of that device relative to the acting direction of gravity. The accelerometer can be connected to an analog or digital readout presenting the angular orientation of the accelerometer relative to the acting direction of gravity. Preferably, the shaft is attached in fixed relation to the accelerometer such that when the accelerometer indicates that there is no angular difference between the reference direction recognized by the accelerometer and the acting direction of gravity, the longitudinal axis of the shaft is parallel to the acting direction of gravity. Accordingly, as the shaft is oriented freely in space, the accelerometer indicates the angular difference (preferably in two planes) between the longitudinal axis of the shaft and the acting direction of gravity.
Operation of this embodiment proceeds as indicated with regard to the first embodiment, with the accelerometer (rather than the fluid-containing enclosure in the first embodiment) indicating when the shaft is in the desired position, that is, when the angular difference between the longitudinal axis of the shaft and the acting direction of gravity matches the cephalad-caudad declination (in the cephalad-caudad plane) and medial angulation (in the medial plane) of the pedicle.
Still another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is similar to that of the second embodiment described above, except that the fluid-containing enclosure of that embodiment is replaced with an accelerometer similar to the accelerometer of the third embodiment described above. Accordingly, when each rotatable mounting is in its zero position, and the accelerometer reads level, the longitudinal axis of the shaft is parallel to the acting direction of gravity. And, accordingly, if either or both of the rotatable mountings are placed in a rotated position, the user can, when the accelerometer is oriented level, read the marks to determine the angular difference between the longitudinal axis of the shaft and the acting direction of gravity.
Operation of this embodiment proceeds as indicated with regard to the second embodiment, with the accelerometer indicating when the accelerometer is oriented level (and thus, if the rotatable mountings have been rotated to match the cephalad-caudad declination and medial angulation of the pedicle, that the shaft is at the desired angulation).